Seismic transducer baseplate and housing assembly

ABSTRACT

A lightweight rigid housing and baseplate structure for a seismic transducer. The baseplate (12) includes a central hub (26) for receiving a lower piston rod (76). Upper and lower skin plates (32 and 36) extend radially outward from the central hub (26). A plurality of equally angularly spaced reinforcing plates (40) also extend radially outward from the hub (26) and have their upper and lower edges rigidly connected to the upper and lower skin plates. A housing assembly (78) rigidly connects an upper piston rod (74) to the baseplate (12), and includes a frusto-conical section (82). The lightweight rigid housing (78) and baseplate (12) structure provides a seismic transducer having in-phase force distribution over substantially the entire area of the baseplate as it is engaged with the earth.

DESCRIPTION BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to transducers for inducing vibrationalsignals in an elastic medium, and more particularly, but not by way oflimitation, it relates to an improved construction for a baseplate andhousing assembly for a transducer especially suitable for generatingrelatively high frequency seismic waves in the earth.

2. Description of the Prior Art

A conventional seismic transducer of the prior art is illustrated inU.S. Pat. No. 3,745,885 to Fair et al., assigned to the assignee of thepresent invention. The Fair et al. device includes a baseplate, areaction mass, and a double-rod-end piston disposed in a cylindricalbore of the reaction mass. The lower rod end is attached to thebaseplate and the upper rod end is attached to an upper frame memberthat is also connected to the baseplate. Seismic transducers such asthat shown in the apparatus of Fair et al. are generally designed foroperation within the range of about 2-80 cycles per second. The presentinvention provides a housing and baseplate structure suitable foroperation in a relatively higher frequency range, up to and exceedingapproximately 250 cycles per second.

SUMMARY OF THE INVENTION

In a seismic transducer designed for relatively high frequencyoperation, there are two primary design parameters which must beoptimized. First, it is necessary that the baseplate and housingstructure be as lightweight as possible. Second, the baseplate andhousing structure must also be kept as rigid as possible so that thedynamic force transmitted into the earth will be in phase over theentire baseplate contact area. The structure connecting the upper end ofthe piston shaft assembly to the baseplate must be of sufficientrigidity that forces transmitted from the upper end of the shaft remainin phase with forces transmitted into the baseplate from the lower endof the shaft.

The present invention achieves such a lightweight rigid housing andbaseplate structure by providing a frame or housing made up of threesections. The first section includes a relatively small diameter flatplate bolted to the upper end of the piston shaft. The outer peripheryof the flat plate is rigidly attached to the second housing sectionwhich comprises a frusto-conical section having a side angle ofapproximately 45° to the axis of the frusto-conical section. The lowerend of the frusto-conical section is in turn rigidly attached to acylindrical barrel section, the bottom end of which is rigidly attachedto the baseplate. The baseplate includes a central hub to which thelower end of the piston shaft is rigidly connected. Upper and lowerparallel skin plates extend radially outward from the upper and lowerends of the central hub and a plurality of stiffener plates extendradially outward from the central hub to provide a rigid baseplateassembly which will maintain in-phase force transmittal and holddownforce distribution across the baseplate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational view with parts in section of the seismictransducer of the present invention.

FIG. 2 is a bottom view of the baseplate of the present inventionshowing the bottom skin plate partially cut away so that the stiffenerplates may be easily viewed.

FIG. 3 is an inverted sectional elevation view of the baseplate assemblyof FIG. 2 taken along line 3--3.

FIG. 4 is an inverted elevational sectional view of the baseplate ofFIG. 2 taken along line 4--4 specifically showing the manner in whichthe stiffener plates and the upper and lower skin plates are assembled.

FIG. 5 illustrates a bottom view of another form of baseplate for use inthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings and particularly to FIG. 1, the seismictransducer apparatus of the present invention includes a baseplate 12having a hydraulically powered vibrator assembly 14 mounted thereon.

The seismic transducer is generally attached to a vehicle 15, such as alarge truck, by means of a plurality of extendable support columns 16and 18 which provide a means for exerting a downward force on baseplate12 to hold it in engagement with a ground surface 20. The extendablesupport columns 16 and 18 are resiliently mounted on baseplate 12 byvibration isolation structure such as airbags 22 and flexible retainingmeans 24, as is well-known in the art.

The construction of baseplate 12 is best shown in FIGS. 2 and 3.Baseplate 12 includes a central hub 26 having an upper end 28 and alower end 30. A first upper skin plate 32 is rigidly attached to hub 26at weld 34. A second lower skin plate 36 is rigidly attached to thelower end 30 of central hub 26 at weld 38. Lower skin plate 36 may bedescribed as being parallel-spaced from upper skin plate 32.

A plurality of reinforcing members or ribs 40 have their inner ends 41welded to central hub 26 and extend radially outward from central hub26. Reinforcing ribs 40 are preferably spaced at equal angles 42 aroundthe axis 44 of central hub 26. This assists in distribution of inphasedynamic forces from both ends of the piston shaft over the entirebaseplate area in contact with the earth.

As best can be seen in FIG. 2, the upper and lower skin plates 32 and 36are substantially rectangular in shape. A plurality of end plates 46,48, 50 and 52 are rigidly connected about the periphery and to the upperand lower skin plates 32 and 36 by any usual means such as by welding.The upper and lower skin plates 32 and 36 and end plates 46, 48, 50 and52 define an enclosed parallel-piped structure. The outer ends ofreinforcing ribs 40 are also rigidly attached to one of the ends plates46, 48, 50 or 52 as by welding, as shown for example at 54. Additionaltransverse reinforcing plates 56 are provided beneath opposite supportcolumns 16 and 18.

The manner of fabricating the upper and lower skin plates 32 and 36, andthe radially extending reinforcing ribs 40, is best illustrated in FIG.4. The upper skin plate 32 is preferably a continuous member asillustrated in FIG. 4, with an upper edge 57 of reinforcing rib 40butted up to the lower surface 58 of upper skin plate 32 and joined bywelds 60.

The lower skin plate 36, however, is preferrably fabricated fromplurality of pie-shaped pieces, 61 and 63 for example, which are spacedabove a lower edge 65 of reinforcing plates 40. The pie-shaped pieces 61and 63 are then welded to upper edges 65 of reinforcing ribs 40 as shownat 67. Slots 71 provide access for plug welding reinforcing plates 56 tothe bottom surface 36.

Referring again to FIG. 1, vibrator assembly 14 consists of a reactionmass 64, to be further described, having a cylindrical bore 66 withinwhich a double-rod-end piston 68 is disposed for reciprocal actuation.Such reaction mass configurations are well-known in the art ofhydraulically driven seismic vibrators.

The double-rod-end piston 68 includes a piston 70 reciprocally disposedwithin a cylinder 72. Upper and lower piston rods 74 and 76,respectively, extend from piston means 70. Lower piston rod 76 has itslower end received within central recess 62 of central hub 26, and issecured to central hub 26 by bolts 77. The upper piston rod 74 isrigidly connected by bolts 79 to a reaction mass housing 78 which may bedescribed as providing a means for rigidly connecting the upper pistonrod 74 to the baseplate 12.

The housing 78 includes three sections 80, 82 and 84, and may also bedescribed as a frame means. The first section 80 may be described as aflat plate section having a radially inner portion 86 rigidly secured toupper piston rod 74 by bolts 79. An outer portion 88 of plate section 80is rigidly connected to second section 82, e.g. by welding, as indicatedat 90. The second section 82 may be described generally as afrusto-conical section having a smaller diameter end 92 and a largerdiameter end 94. Frusto-conical section 82 projects an angle 96 with aline parallel to central axis 44 of double-rod-end piston assembly 68.The angle 96 may vary with different designs, but preferably it is inthe range of 40°-50° and optimally about 45°.

As the angle 96 is increased towards the maximum of 90° the constructionof the frusto-conical section 82 approaches that of a complete flatplate section. A major problem with a completely flat section is that toachieve the necessary rigidity for high frequency operation that flatsection would have to have a much thicker cross section andcorresponding higher weight than is necessary with the frusto-conicalsection of the present invention. By providing the frusto-conicalsection 82, the forces from the upper piston rod 74 are transmitted to alarge extent through compressional and tensional forces in therelatively thin plate making up frusto-conical section 82, as opposed totransmission almost entirely by bending forces within a flat platesection.

On the other hand, the angle 96 should not be made too small, because asthe angle 96 decreases below 45° the necessary diameter of flat plate 80increases. If the angle 96 were ultimately reduced to 0°, the designwould once again have degenerated to that of a flat top section with adiameter approaching that of cylindrical section 84. It is for thesereasons that the angle 96 is optimally chosen as approximately 45°.

The third section 84 of housing 78 is a generally cylindrical sectionincluding a cylinder 98 having upper and lower flanges 100 and 102,respectively. A first plurality of radially spaced reinforcing plates104 are connected between an outer surface 106 of cylinder 98 and theupper end flange 100. Plates 104 reinforce flange 100 to prevent anysignificant radial expansion of cylindrical section 84, while alsodistributing stress from the flange weld sections. A second plurality ofradially spaced reinforcing plates 108 are connected between outersurface 106 and the lower flange 102. These second plates 108 are ofvarying lengths so that the forces imposed at plates 108 on cylindricalsection 84 are optimally distributed along the length of cylindricalsection 84. Plates 108 also serve to dampen internal resonances withincylindrical section 84.

The upper flat plate section 80 of housing 78 would also include portpassages, e.g. passage 110 disposed therethrough which connects a portpassage 112 in piston rod assembly 68 from the lower end of cylinder 72to a servo-valve assembly 114, and this may be described generally as asource of hydraulic fluid under pressure. Similar port means within rodassembly 68 (not shown) connect servo-valve 114 with the upper end ofdrive cylinder 72, i.e. above piston 70. Still other ports through platesection 80 and rod assembly 68 provide hydraulic fluid flow for the massbiasing and seal bleed-back functions as are well-known in the art, suchports not being specifically shown.

The reaction mass assembly 64 has a shape similar to the internal volumeof housing 78 and is defined by a cylindrical portion 116 and afrusto-conical portion 118 extending from an end of cylindrical portion116.

FIG. 5 illustrates alternative structure which consists of a round plate32a, lower skin plate 36a, radially extending reinforcing plates 40a,and a circular end plate 46a. To achieve uniform, in phase, forcedistribution across a baseplate, the circular design of FIG. 5 may bepreferable to the rectangular design of FIG. 2. Other considerations,such as the ease of mounting the seismic transducer 10 on the vehicle15, may however dictate the use of a rectangular baseplate.

The seismic transducer, and particularly the baseplate 12 and housing orframe 78, are specifically designed for operation within a widefrequency range up to and exceeding 250 cycles per second. In designinga transducer for operation in the higher seismic frequency bands, i.e.above about 80 cycles per second, an important design parameter is thenatural frequency of the spring-mass system represented by the mass ofthe baseplate 12 and housing 78, and the spring constant of the trappedhydraulic fluid between servo-valve 114 and piston 70. That naturalfrequency should be above the operating frequency range.

This natural frequency is inversely proportional to the square root ofthe mass of baseplate 12 and housing 78, therefore the natural frequencycan be raised by reducing the weight of those components.

Additionally, higher frequency operation increases the need for a rigidbaseplate and housing to assure that forces transmitted through thebaseplate and housing into the earth remain substantially in phase. Itis desirable that there be a phase difference of no more than 90°, andpreferably considerably less than that, across the entire area ofbaseplate 12 contacting the earth. Of course, the baseplate and housingmust also be sufficiently strong to withstand the transmitted forceswithout structural failure.

Such a lightweight, rigid and strong baseplate and housing structure isprovided by the present seismic transducer. In the operating range ofabout 250 cycles per second the displacement of baseplate 12 may be onthe order of 0.006 thousandths of an inch peak to peak, depending on theearth medium under the baseplate. To maintain in-phase transmission offorces through housing 78, the allowable deflection within housing 78 ison the order of 1/10 or less of the displacement of baseplate 12. Thenecessary rigidity of housing 78 to achieve this small deflection isprovided by the structure previously described, while a relatively lowweight is also maintained. The necessary strength of housing 78 isprovided through the use of high tensile steel. Similarly, thedeflection within baseplate 12 should also be on the order of 1/10 orless of the displacement of baseplate 12.

Thus it is seen that the seismic transducer baseplate and housingassembly of the present invention achieves the objects and advantagesmentioned as well as those inherent therein. While presently preferredembodiments have been described for the purpose of this disclosure,numerous changes and modifications may be made to those embodiments,which changes and modifications are included within the scope and spiritof the present invention as defined by the following claims.

The embodiments of this invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A transducer apparatusfor inducing waves in an elastic medium, comprising: a baseplate meansfor engaging a surface of the elastic medium; a reaction mass having acylinder bore extending therethrough; a double-rod-end piston, disposedin said bore so that said reaction mass is driven in reciprocationrelative to said piston, a first rod end of said piston being rigidlyconnected to said baseplate means; and housing means for rigidlyconnecting a second rod end of said piston to said baseplate means, saidhousing means including a frusto-conical section having a smallerdiameter end rigidly connected to said second rod end and a largerdiameter end rigidly connected to said baseplate means.
 2. The apparatusof claim 1, wherein said frusto-conical section comprises an angle inthe range of about 40°-50° with a longitudinal axis of said piston. 3.The apparatus of claim 1, wherein said housing means further comprises acylindrical section having a first end rigidly connected to saidbaseplate means and a second end rigidly connected to said largerdiameter end of said frusto-conical section.
 4. The apparatus of claim3, wherein said cylindrical section includes: a cylinder; first andsecond radially outward extending flanges connected to said cylinder atsaid first and second ends, respectively, of said cylindrical section;and first and second pluralities of radial reinforcing plates connectedbetween an outer surface of said cylinder and said first and secondflanges, respectively.
 5. The apparatus of claim 4, wherein saidreinforcing plates of said first plurality of reinforcing plates are ofvarying lengths.
 6. The apparatus of claim 3, wherein saidfrusto-conical section comprises an angle of approximately 45° with itslongitudinal axis.
 7. The apparatus of claim 3, wherein said housingmeans further comprises a plate section having a radially inner portionrigidly connected to said second rod end of said piston, and having anouter portion rigidly connected to said smaller diameter end of saidfrusto-conical section.
 8. The apparatus of claim 7, wherein said platesection includes port means for communicating a source of hydraulicfluid under pressure to ports located in said double-rod-end piston. 9.The apparatus of claim 3, wherein said reaction mass has a shape definedby a cylindrical portion with a frusto-conical portion extending from anend of said cylindrical portion.
 10. The apparatus of claim 9, whereinsaid frusto-conical portion of said reaction mass comprises an anglewith its longitudinal axis substantially equal to the angle of saidfrusto-conical section of said housing means.
 11. The apparatus of claim1, wherein said baseplate means comprises: a hub means rigidly connectedto said first rod end of said piston; first and second spaced parallelplates extending radially outward from said hub means; and a pluralityof reinforcing plates extending radially from said hub means, saidreinforcing plates being rigidly attached to said hub and each of saidfirst and second spaced plates.
 12. The apparatus of claim 11, whereinsaid radially extending reinforcing plates are equally angularly spacedabout the axis of said hub means.
 13. The apparatus of claim 11, whereinsaid baseplate means is further characterized as being a circularbaseplate means.
 14. The apparatus of claim 11, wherein said baseplatemeans is further characterized as being rectangular and as includingfirst, second, third and fourth end plates rigidly connecting said firstand second spaced plates to define an enclosed parallel-piped.
 15. Theapparatus of claim 1, wherein said housing means further comprises aplate section having a radially inner portion rigidly connected to saidsecond rod end of said piston and having an outer portion rigidlyconnected to said smaller diameter end of said frusto-conical section.16. A seismic transducer apparatus comprising: a baseplate means forengaging a surface of the earth and inducing seismic waves therein, saidbaseplate means being so constructed that dynamic forces transmittedfrom the transducer to the earth are substantially in phase over theentire baseplate area in contact with the earth, said baseplate meansincluding: a central hub; first and second spaced skin plates extendingradially outward from said central hub; and a plurality of reinforcingmembers extending radially from said hub, said reinforcing members beingrigidly attached to said first and second spaced skin plates; a reactionmass having a cylinder bore extending therethrough; a double-rod-endpiston, disposed in said cylinder bore so that said reaction mass ishydraulically driven in reciprocation relative to said piston, a firstrod end of said piston being rigidly connected to said central hub; andframe means for rigidly connecting a second rod end of said piston tosaid baseplate means.
 17. The apparatus of claim 16, wherein said framemeans is further characterized as being a rigid frame means having astructural deflection less than about 1/10 of a displacement of saidbaseplate means at the high frequency end of the designated frequencyrange.
 18. The apparatus of claim 16, wherein said baseplate means has astructural deflection less than about 1/10 of said displacement of saidbaseplate means at the high frequency end of the designated frequencyrange.
 19. The apparatus of claim 16 or 17, wherein said frame meanscomprises a frusto-conical section having a smaller diameter end rigidlyconnected to said second rod end and a larger diameter end rigidlyconnected to said baseplate means.
 20. The apparatus of claim 16,wherein said frame means comprises: a plate section rigidly connected tosaid second rod of said piston; a frusto-conical section having asmaller diameter end rigidly connected to said plate section; and acylindrical section having a first end rigidly connected to saidbaseplate means and a second end rigidly connected to a larger diameterend of said frusto-conical section, said frame means being sufficientlyrigid that forces transmitted from said second rod end of said piston,through said frame means, to said baseplate means are substantially inphase with forces transmitted from said first rod end of said piston tosaid baseplate means.
 21. The apparatus of claim 16 which furtherincludes: hold down means bearing upon opposite sides of said baseplatemeans first and second spaced skin plates; and transverse plate meansdisposed between said first and second spaced skin plates on saidopposite sides of said baseplate means.